Added Maxim DS32kHz support

As described in the Adafruit Clocks forum, the Maxim DS32kHz may be
used with the Ice Tube Clock to increase the accuracy of timekeeping.

http://forums.adafruit.com/viewtopic.php?f=41&t=14941

This feature may be enabled by defining the EXTERNAL_CLOCK macro in
config.h.  Doing so, allows the ATmega328p to accept an external clock
signal on XTAL1 instead of the usual crystal oscillator.  Also,
defining this macro raises the BOD threshold voltage from 1.8 to 2.7,
because the DS32kHz requires 2.7 volts for timekeeping.

I also moved the ADAFRUIT_BUTTONS macro from the Makefile to config.h,
where future compile-time options belong.

Finally, the Makefile no longer defines fuse bits.  Instead, fuse bits
and lock bits are defined in the code (icetube.c).